Gearing

[realsteelGM]

Anyone familiar with bringing R.P.M.'S down? I am running a complete rebuild 283 with sm420, 3:54 dana rear diff. and 15" tires. Looking to bring down r.p.m.'s from 2336 to 1700 @ 55 m.p.h. What is the best way to go about this?

[blackedoutharley]

Taller tire (although it wont get you as far as you want), Gear Vendors overdrive unit, other overdrive trans swap..

Going to a 31" tall tire and swapping to 3.08 gears will get you to down to 1850 or so.

Switching to a 31" tall tire alone without changing the trans or rear gears will get to you about 2100

[realsteelGM]

How about a 5 speed NV4500 transmission would it give better gearing through all 5 speeds?

[realsteelGM]

Originally Posted by blackedoutharley

Taller tire (although it wont get you as far as you want), Gear Vendors overdrive unit, other overdrive trans swap..

Going to a 31" tall tire and swapping to 3.08 gears will get you to down to 1850 or so.

Switching to a 31" tall tire alone without changing the trans or rear gears will get to you about 2100

How about changing the transmission to a 5 speed?

[blackedoutharley]

Changing the transmission to the NV4500 would probably give you what you want but it is easier, and cheaper, to install a taller tire and/or different gears.

[realsteelGM]

Originally Posted by blackedoutharley

Changing the transmission to the NV4500 would probably give you what you want but it is easier, and cheaper, to install a taller tire and/or different gears.

I really don't want to sacrifice my posi rear diff. Can I just swap out the gears?

[blackedoutharley]

Yes... swapping gears would probably be easier with no mods required compared to swapping to a TKO or other overdrive manual trans.

A TKO with a 28" tall tire and 3.54 gears is just under 1600 rpm @ 55 mph.

There are gears available for your Dana 44 in both 3.08 ratio as well as 2.73.

The Dana 44 with 2.73 gears and a 28" tire spins the engine @ 1800 rpm @ 55mph although I am certain that depending on what engine you have you will not be happy with how it reacts from a dead stop... something with this tall of a gear would need to have a heavy flywheel to build up enough inertia to get it moving.

[realsteelGM]

Originally Posted by blackedoutharley

Yes... swapping gears would probably be easier with no mods required compared to swapping to a TKO or other overdrive manual trans.

A TKO with a 28" tall tire and 3.54 gears is just under 1600 rpm @ 55 mph.

There are gears available for your Dana 44 in both 3.08 ratio as well as 2.73.

The Dana 44 with 2.73 gears and a 28" tire spins the engine @ 1800 rpm @ 55mph although I am certain that depending on what engine you have you will not be happy with how it reacts from a dead stop... something with this tall of a gear would need to have a heavy flywheel to build up enough inertia to get it moving.

Looks like you know your gearing, seeing as i didn't tell you that the 3:54's were a dana 10 bolt diff.
Looks like the 3:08's would be a good choice combined with the taller wheel. Yet torque issues are evident already with the 283 which has great power at high r.p.m. but needs down shifting on the hills with a load. Here is a pic of my engine if I can upload it to ya. This is a 1958 corvette or truck block.

[blackedoutharley]

Remember that everything is combined to work together; engine and its torque output (which is related to size, camshaft, compression etc), transmission, clutch, rear gearing & overall tire diameter.

If one thing is out it will make the overall results less than admirable. I would probably just go to a larger tire. I assume we are talking about the truck in your profile picture... If so, you might try installing a taller tire first. 31X10.50R15 is a good size that, depending on what wheels you have on the truck, is a great fit.

[realsteelGM]

Still in break-in mode on this engine, it has some timing issues that still need tweaking, and compression is only 100 lbs across all 8 cylinders, tells me that rings are not seated fully yet, quench values checked out good. Any idea on time frame or special techniques for rings to seal? Everything is new on this engine including heads, vortec swirl intake with 1.94 and 1.50 valves, 500cfm carb, mild cam, slightly over stock. May just need some more tweaking before I spend more money. I do think your right on the tires and diff before spending time and money on a overdrive. I looked up the gear vendor overdrive set up, interesting, but need more info and reviews on it. Bottom line, only getting 13-15m.p.g. on highway, that was one of the main reasons I decided to build the 283, for fuel mileage and reliability. I'ts still been a challenge and a lot of fun building this unit.

[Longhorn Man]

if you drop your RPMs to 1800 at freeway speed, your MPG will probably drop

[realsteelGM]

How do you equate losing m.p.g. to 1800 r.p.m.?

[bfay17]

The motor will be out of its rpm band and will have to work harder to just cruise which will burn more fuel. Now thats not always the case but it could happen.

[GASoline71]

A 283 will work harder to lug that heavy truck around as well.

283's are short stroke, high winders. It won't be happy luggin' down under 2,000 rpm's.

Gary

[bfay17]

I didnt think it would like it to much thanks for confirming that for me.

[Captainfab]

Lower rpm's doesn't always mean better mpg's. As mentioned above, the engine needs to be in it's power band.

[realsteelGM]

Originally Posted by GASoline71

A 283 will work harder to lug that heavy truck around as well.

283's are short stroke, high winders. It won't be happy luggin' down under 2,000 rpm's.

Gary

Your right on that note, my main concern is fuel mileage on the highway. I has the muncie 420sm and matched gearing in the diff. along with the original engine the truck came with. So everything is matched except performance cam, vortec heads, roller rockers.

[cableguy0]

If you run that 283 under 2k rpm on the highway you will lose mpg. Even on the highway the engine will be lugging. You will end up with a transmission that searches in and out of overdrive if you go that route. Your probably pretty close to being in the sweet spot now. Have you driven the truck yet to know what kind of mileage its getting?